Project Summary and Abstract Whereas treatment of low-density lipoprotein cholesterol (LDL-C) with statins reduces risk for coronary heart disease (CHD), a significant residual risk remains. This observation raises the following fundamental question: beyond LDL-C, which lipid pathway(s) affects risk for CHD in humans? We have accrued preliminary evidence showing that the lipoprotein lipase (LPL) pathway contributes to the development of CHD in humans. Common, low-frequency, and/or rare DNA sequence variants in lipoprotein lipase (LPL) and two genes encoding LPL- regulating proteins (APOA5, APOC3) are associated with risk of CHD as well as plasma triglycerides (TG). For example, through exome sequencing, we discovered that 1 in 150 individuals carried one of four rare apolipoprotein C-III (APOC3) mutations, each leading to loss-of-function (LoF) (Crosby*, Peloso*, N Engl J Med, in press). Carriers of APOC3 LoF mutations had lower plasma TG and apoC-III protein level as well as 40% lower risk for CHD (P = 4 x 10-6). These findings indicate that beyond LDL-C, the LPL pathway is a key route to CHD and suggest several questions: (1) what is the full suite of genes that regulate/interact with LPL and comprise the LPL pathway?; (2) are there other genes in the LPL pathway with rare, cardio-protective alleles?; and (3) can we study physiology in APOC3 protective allele carriers to understand mechanisms behind the protection? To address these questions, we propose the following specific aims: Aim 1: To test the hypothesis that additional genes in the LPL pathway can be discovered using computational approaches and coding variation in these new genes will associate with plasma TG; Aim 2: To test the hypothesis that LPL pathway genes (beyond APOC3) harbor rare, LoF alleles that protect against CHD; and Aim 3: To test the hypothesis that APOC3 LoF mutation carriers have increased lipolysis of TG-rich lipoproteins and improved insulin sensitivity. Furthermore, the PI has assembled a mentoring committee who will provide the necessary training and support to accomplish the proposed research, as well as facilitate the growth of the PI. The proposed research will train a young investigator in 3 key areas: (1). To expand her knowledge of lipoprotein metabolism; (2). To develop skills for performing and interpreting pathway analysis; and (3) To perform hypothesis-driven human physiology experiments. Successful completion of the training plan and the proposed research should propel the PI to independence as a biomedical investigator.